THERMIONIC VACUUM TUBES 



49 



an inductance of such a value as to make the impedance between 

 grid and plate infinite at this frequency. 



IV. Thermionic Amplifiers 



Equation 4, given in Sec. 12 is of fundamental importance in the 

 design of vacuum tube amplifier circuits. Neglecting the second 

 term on the right hand side which, as previously pointed out, expresses 

 distortional effects and should therefore be very small in amplifier 

 circuits, the equation can be written 



ix e 



J 



(6) 



Z+r p ' 



in which the impedance Z = r-\-j x is substituted for r. From Equation 

 6 both the voltage and power amplification of a tube for any particular 

 circuit can readily be calculated. 



14. Voltage Amplification. Assuming as above that the tube works 

 into an output impedance Z, it follows that the voltage amplification 

 (i.e., the ratio of the output to the input voltage) is 



mZ 

 Z + rp 



This expression shows that the voltage amplification increases as Z 

 increases. Considering separately the two cases in which Z is a 

 pure resistance and pure reactance, typical values of the voltage 

 amplification are plotted in Fig. 21. Curve a corresponds to reactance 



9 



Fig. 21 



in the output and b to resistance. These curves show that the voltage 

 amplification rises much more rapidly when reactance is used, reaching 



x 



90% of its maximum value when 



r* 



If the resistance component of Z is made as small as possible, E P 

 becomes practically equal to the potential across the plate battery, 



